An X-ray CT system used in a medical institution or the like photographs the internal structure of a subject by applying an X-ray to the subject. Specifically, the X-ray CT system includes an X-ray irradiation source and a radiation detector which is arranged to be opposed to the X-ray irradiation source through a subject and which has an X-ray detection section arranged in a one-dimensional array. The detection section functions to convert a received X-ray into an electrical signal and includes a scintillator which converts the X-ray into a visible light beam and a photodiode which converts the visible light beam into an electrical signal. This radiation detector receives the X-ray which passes through the subject and records the electrical signal obtained based on the received X-ray. While maintaining the positional relationship between the X-ray irradiation source and the radiation detector, the X-ray CT system variously changes an X-ray irradiation angle and repeatedly receives X-rays. The X-ray CT system then conducts processings such as convolution and back-projection to the electrical signals thus obtained, thereby reconstructs the images of the cross-sections (which will be referred to as “slices” hereinafter) of the subject through which the X-rays pass.
Recently, in particular, the development of a multi-slice X-ray CT system which can simultaneously photograph a plurality of slices by one X-ray irradiation is actively underway. The multi-slice X-ray CT system has a plurality of X-ray detection sections, each of which is in the form of an array, which are arranged to correspond to a plurality of slices, respectively, collects X-rays which pass through the respective slices and reconstructs slice images. The multi-slice X-ray CT system is, therefore, required to include a radiation detection apparatus in which each detection section is arranged not in a one-dimensional array but in a two-dimensional array. Photodiodes which constitute each detection section should be arranged two-dimensionally, accordingly.
A conventional radiation detector shown in FIG. 24A (“conventional art 1”) is formed by arranging a plurality of single-slice one-dimensional photodiode arrays 102 in parallel on a substrate 101, arranging photodiodes 103 two-dimensionally, and mounting two-dimensional scintillator arrays each having scintillator elements corresponding to the photodiodes on the photodiodes 103, respectively. The first pad 104 of each photodiode 103 is electrically connected to the second pad 105 which is provided on the substrate 101 by a bonding wire 106. An electrical signal output from each photodiode 103 is propagated on a wiring provided on the substrate 101 and output to the outside of the substrate 101.
There is also known a structure in which a plurality of photodiodes distributed in a matrix and wirings corresponding to the photodiodes are integrally formed on a single semiconductor substrate (which structure will be referred to as “conventional art 2” hereinafter). A radiation detector is formed by mounting a two-dimensional scintillator array which includes scintillators corresponding to the respective photodiodes, on the semiconductor substrate on which the photodiodes are thus incorporated.
The conventional art 1 has the following disadvantages. First, according to the conventional art 1, the one-dimensional photodiode arrays 102 are arranged on the plane substrate 101. A difference in height is, therefore, disadvantageously generated between second pads 105 provided on the substrate 101 and the photodiodes 103 by as much as the thickness of each photodiode array 102. If the bonding wires 106 are provided in a state in which such difference is generated, it is necessary to separate the position of each second pad 105 from each photodiode 103 by a predetermined distance in horizontal direction as shown in FIG. 24B. If the position of the second pad 105 is thus separated from the photodiode 103, however, the distance between the photodiode arrays 102 widens. As a result, an area occupied by the photodiodes relative to the entire radiation detector becomes small and the X-ray receiving sensitivity of the detector disadvantageously deteriorates.
In addition, it is necessary to accurately locate the one-dimensional photodiode arrays 102 on the substrate 101. To manufacture such a radiation detector as the conventional art 1, therefore, it is disadvantageously necessary to newly provide a mounting device which fixes the one-dimensional photodiode arrays onto the substrate 101 or to use a special positioning tool.
Moreover, since the conventional art 1 has a structure in which the two-dimensional scintillator arrays are directly arranged on the plural one-dimensional photodiode arrays 102, the conventional art 1 has disadvantages of a small contact area and lowered mechanical strength.
The conventional art 2 has disadvantages, as well. All the photodiodes are mounted on the single semiconductor substrate. For that reason, if even one defective photodiode exists among the photodiodes which constitute the two-dimensional photodiode array, the radiation detector cannot be formed, with the result that the other photodiodes mounted on the semiconductor substrate must be abandoned. The individual photo diodes which constitute the two-dimensional photodiode array are required to be arranged two-dimensionally. A redundant circuit as employed in a DRAM cannot be, therefore, used and yield is disadvantageously quite low with the structure of the conventional art 2.
Further, the conventional art 2 has a structure in which necessary wirings are also mounted on the semiconductor substrate. These wirings are provided to correspond to the photodiodes, respectively. Therefore, as the number of the photodiodes increases, that of the wirings increases. If the number of slices increases, in particular, the number of wirings necessary to output electrical signals to the outside of the substrate increases. To suppress a decrease in an area occupied by the photodiodes on the semiconductor substrate, it is necessary to narrow the width of each wiring. However, if the wirings are narrower, electrical resistance disadvantageously increases and the probability of breaking the wirings disadvantageously increases.
It is noted that some of these disadvantages explained above are not limited to the multi-slice radiation detector but seen in a single-slice radiation detector. If one-dimensional photodiode arrays are arranged on a substrate, for example, the difference in height between pads and photodiodes is disadvantageously generated.
Moreover, these disadvantages occur not only to the radiation detector but also ordinary photo-detectors. Normally, a photo-detector has a structure of the radiation detector from which scintillators are excluded and the same as the radiation detector in that the two-dimensional photodiodes are arranged. To improve the light receiving sensitivity of the photo-detector, it is preferable that an area occupied by the photodiodes on the substrate is large. However, because of the same disadvantages as those explained above, there is no avoiding narrowing the light receiving area.